Originally Posted by
Cameron Wood
My approach is to fit rigid foam sheet panels between the rafters, leaving an air gap above.Fiberglass can fill the remaining space, covered in your case even by Tyvek.
Vents are added at the eaves and ridge which exhausts heat and moisture. The shiny foil face helps as a radiant barrier.
I would leave 1/2 or 5/8" space (furring strips), cut the probably 1 1/2" foam into 4' butts with drywall square and snap-off type knife with a long blade, for a press fit into the bays.
I've done this quite a few times. It is DIYable, relatively inexpensive, reversable/modifiable, and does not promote moisture or heat degradation, and is not ugly.